1. Field of the Invention
The present invention relates to a receptor layer forming resin for a thermal transfer image receptor sheet having excellent dyeing property, light resistance, mold-releasing property, etc. and to a thermal transfer image receptor sheet using the same.
2. Description of the Related Art
As a method for forming an image by utilizing thermal transfer, a method (sublimation type thermal transfer system) is known, in which a full color image is formed by laminating: a thermal transfer sheet obtained by supporting a substrate sheet, such as paper or plastic film, with a sublimation dye as a recording material; and a thermal transfer image receptor sheet obtained by providing a receptor layer for the dye on paper or a plastic film. Because this method uses a sublimation dye as a coloring material, density and gradation can be freely controlled by unit of a dot, so that a full color image exactly according to a manuscript can be expressed on the image receptor sheet. Therefore, it is applied to the formation of a color image in digital cameras, videos, computers and the like. The image has high quality which stands comparison with that of a silver salt photograph.
In order to obtain a high quality image and the like on a thermal transfer image receptor sheet, the receptor layer must have excellent dyeing property, light resistance, mold-releasing property and the like. With regard to the dyeing property, it is usually possible to obtain a receptor layer having excellent dyeing property by using a resin having a relatively low glass transition temperature. However, the resin having a low glass transition temperature has a problem that it is easily melted by a heat in a transfer operation and fused to other members.
On the other hand, by using a resin having a relatively high transition temperature, a receptor layer having excellent mold-releasing property, light resistance, and the like can be obtained. Though such a receptor layer does not give rise to the problems described above such as the fusion, a resin having a high glass transition temperature has a problem that it is difficult to be softened during a thermal transfer operation so that it is not very much dyed with a dye, which makes it difficult to obtain a high quality image and the like.
To deal with such problems, using of a mixture, of the above-described resins having a low glass transition temperature and the resin having a high glass transition temperature, is proposed. For example, Japanese Patent Application Laid-Open (JP-A) No. 6-79974 discloses a receptor layer using a resin having a glass transition temperature of −100 to 20° C. and a resin having a glass transition temperature of 40° C. or higher. In a method disclosed in JP-A No. 6-79974, the above two types of resins are mixed to form a receptor layer. In this case, however, there is a problem that the glass transition temperature of the resulting receptor layer will be an average of the two. Therefore, the effect of the resin having low grass transition temperature, that excellent dyeing property can be imparted, and the effect of the resin having high glass transition temperature, that excellent mold-releasing property and light resistance can be imparted, are offset.